[GreenKeys] Problem with Selector Magnet Driver Circuit on ASR33

[Clay Archer]

Yes, that's normal for it to get hot.  It's live even when switched "off".
You can find a schematic and parts list at www.bitsavers.org/pdf/teletype .
R7 should also be getting hot.

 

You can secure the resistor by wrapping and soldering a piece of bare wire
around the leg on the top side of the board so the resistor doesn't push
through.  Solder a piece of wire across any traces that are broken or
lifted.  The idea is to make the lifted trace thicker with wire and solder.
You could glue the trace back onto the board but it's not necessary.  I
suggest you unplug the teletype when not in use, or put it on a switched
plug strip.

 

Clay

 

 



 

  _____  

From: greenkeys-bounces at mailman.qth.net
[mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Rob Jarratt
Sent: Sunday, January 13, 2013 7:13 AM
To: greenkeys at mailman.qth.net
Subject: [GreenKeys] Problem with Selector Magnet Driver Circuit on ASR33

 

I have noticed that when I plug in my ASR33 one of the parts becomes very
hot, even when the ASR33 is switched off. I have located the source of the
heat as a circuit board because there are tell-tale heat marks on the board
and I can feel the heat there too. The heat marks correspond to the large
area of solder on the back of the board, and the large resistor (R4) looks
mottled and has no markings (possibly burned off?). Oddly, the large solder
area does not appear to be connected to anything, I checked with a
multimeter. From the markings on the metal plate that covers the board it
would seem that R4 is getting very hot, the markings on the board suggest
that R2, R5, R6 and R8 are also getting hot.

On the back the board is marked 181823 and on the front it is marked 181821.
The former part number is not mentioned in the parts document, the latter
seems to be the selector magnet driver circuit and the description seems to
correspond, so I think it is right. If that is the case R4 is 0.82ohm, but I
don't know its power rating.

Also, while examining the components I moved one of them, when I did so the
track on back became detached from the board, although the track is still
intact.

So I have a few questions:

Should this board be live when the ASR33 is plugged in, but not switched on?

Why might there be heat marks associated with the large soldered area on the
back, when it is not connected to anything?

What are the power ratings of the large resistors R2, R4, R5, R6 and R8?

What might cause R4 and possibly also R2, R5, R6 and R8 to get hot?

What can I do to reattach the detached track to the board?

Regards

Rob

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